1. Field of the Invention
The present invention generally relates to an image processing apparatus and, more particularly, to an image processing apparatus which processes pixels by a single instruction multiple data stream (SIMD) control method and removes an isolated point included in an image.
2. Description of the Related Art
Many printed materials have an isolated point (or an isolated dot) which is not a real part of an image or a character printed on a recording material. Such an isolated point may be undesirably formed on a recording material, for example, due to scattering of toner in a printing process. If a copy is taken based on the printed material having such an isolated point for a plurality of times, the isolated point may be enhanced by the copying operations which may result in deterioration of an image quality.
Accordingly, methods for removing such an isolated point have been suggested. Japanese Laid-Open Patent Application No. 9-6917 discloses a method for detecting an isolated point based on a run length of a white pixel run and a black pixel run. Hereinafter, this method is referred to as a run length method. Japanese Laid-Open Patent Application No. 8-305844 discloses a method for detecting an isolated point based on intensity of pixels forming an area surrounding a currently processed pixel. Hereinafter, this method is referred to as an area method.
In a conventional circuit for performing the run length method or the area method, single-pixel data is supplied for each pixel clock cycle. FIG. 1 is a bock diagram of an image processing unit provided in a conventional copy machine. In FIG. 1, pixel data corresponding to a single pixel is supplied from a scanner 201 to the image processing unit 202 for each cycle of a pixel clock signal.
The image processing unit 202 comprises a filtering unit 203, an isolated point removing unit 204, a is elector 205, a gamma processing unit 206 and a gradation processing unit 207. The pixel data generated by the scanner 201 is supplied to both the filtering unit 203 and the isolated point removing unit 204 so as to simultaneously performing a filtering process and an isolated point removing process. An output of the filtering unit 203 and an output of the isolated point removing unit 204 are selected by the selector 205. The selected output is supplied to a printer (not shown in the figure) via the gamma processing unit 206 and the gradation processing unit 207.
The filtering unit 203 performs an enhancement process and a smoothing process on the pixel data, and selectively outputs one of the enhanced pixel data and smoothed pixel data to the selector 205.
Recently, the single instruction multiple data stream (SIMD) control method has become used in a copy machine. In the SIMD control method, data corresponding to a plurality of pixels is supplied at the same time, and the plurality of pixels are processed simultaneously. That is, for example, pixel data corresponding to one scan line, which may include pixel data of a few hundreds pixels, is simultaneously processed by a single instruction so as to reduce a processing time.
The above-mentioned run length method can be used with a small memory capacity and is suitable for a process in which the pixel data corresponding to a single pixel is processed one after another. However, the run length method cannot be effectively used with the SIMD control method since the run length method is based on the process in which a single-pixel data is supplied and processed one after another.
In the above-mentioned area method, the currently processed pixel data is compared with pixel data corresponding to the pixels surrounding the currently processed pixel. Accordingly, the area method cannot be used with the SIMD control method since if the area method is used with the SIMD control method, the pixel data being processed must be compared with pixel data which corresponds to the pixels included in the same scan line and is also being processed at the same time.
As mentioned above, the conventional isolated point removing methods are not suitable for an image processing apparatus such as a copy machine which uses the SIMD control method to increase a processing speed.